1. Field of the Invention
The present invention relates to an anode active material containing silicon, an anode using it, and a battery using it.
2. Description of the Related Art
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery can provide a higher energy density compared to a lead battery and a nickel cadmium battery. The lithium ion secondary battery has a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector.
As an anode active material contained in the anode active material layer, a carbon material such as graphite has been widely used. In recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
However, when silicon having the high theoretical capacity is used as an anode active material, the anode active material inserting lithium when charged is highly activated. Thus, the electrolytic solution is easily decomposed, and lithium is easily inactivated. Further, since the anode active material inserting lithium is significantly expanded and shrunk, the anode active material layer is easily dropped. Accordingly, though a high capacity is obtained, sufficient cycle characteristics are hardly obtained.
Therefore, to improve the cycle characteristics as well when silicon is used as the anode active material, it is suggested to control the crystal state of silicon. Specifically, a technique that the peak intensity ratio based on Raman spectroscopic analysis (in the vicinity of 480 cm−1/in the vicinity of 520 cm−1) is set to 0.05 or more has been known (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-083594). Further, a technique that the surface of silicon particles are covered with non-graphitic carbon material, and the peak intensity ratio of silicon to carbon based on X ray Electron Spectroscopy for Chemical Analysis (ESCA) is set to in the range from 0 to 0.2 has been known (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-259475).